Urethane modified nylon magnet wire enamel

ABSTRACT

Disclosed is a magnet wire composition comprising the reaction product of a polyamide resin, a titanate catalyst, and a solvolytic agent admixed with a blocked polyisocyanate and hydroxylated polyester. Also disclosed is a method of applying such magnet wire enamel to a magnet wire substrate and additionally disclosed is a method for making such magnet wire enamel.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 945,160 filed on Dec. 22,1986 now U.S. Pat. No. 4,725,458.

This application relates to commonly assigned, U.S. Pat. No. 4,716,190for A METHOD FOR EQUILIBRATING POLYAMIDE MAGNET WIRE COATINGS AND ENAMELCOMPOSITIONS by F. A. Lavallee filed on even date herewith.

TECHNICAL FIELD

The technical field of art to which this invention pertains is polyamidecoating compositions and particularly nylon coating compositions oninsulated magnet wire substrates.

BACKGROUND ART

Magnet wire enamels constitute unique insulating materials. Theseenamels are applied to wire substrates as an uncured resin layer whichis then cured by exposing it to elevated temperatures. These insulatinglayers have typical thicknesses ranging in tenths of a mil and theentire insulating layer may comprise a plurality of individually appliedand cured layers of the same or different resins. These insulatinglayers must possess certain qualities such as high dielectric strength,excellent flexibility, thermal stability and must be capable of uniformapplication to the wire substrate.

In the past, enamels have been prepared from polyamide resin materialsor modified polyamide resins, however, many of these enamels had to beprepared as low solids materials to maintain the desired viscosity whichwould allow easy and uniform application to the wire. Since presently itis desirable to increase the solids content of these enamels, therebylowering the amount of solvents used and thereby the cost of the enamel,such polyamide resins have been equilibrated in a reaction between thepolyamide and a monoethanolamine as shown in reaction 1: ##STR1## Thisreaction requires that an excess of monoethanolamine be present duringthe reaction. The excess must be neutralized prior to the coating beingapplied to the wire and is accomplished by adding acetic anhydride tothe solution which reacts with the monoethanolamine in the primaryreaction as shown in reaction 2 below: ##STR2## However certainadditional side reactions also occur during this neutralization process,the most important of which is shown below in reaction 3. ##STR3## Thisreaction being reversible at relatively low temperatures (about 80° C.to about 100° C.) means that during the oven curing of the coating(which temperatures exceed those to reverse the reaction) acetic acidand monoethanolamine are produced. The acetic acid is easily volatilizedout of the coating during curing but the ethanolamine is not completelyremoved and remains behind. This material is known to be deleterious tomany wire varnish curing mechanisms. Therefore, the use of this nylon asa wire coating (particularly for fine wires) results in wire havingundercured coatings which do not effectively adhere to the wire or towire whose mechanical and electrical properties are prematurely reducedduring use.

In addition, the resulting wire coatings do not exhibit uniformproperties from batch to batch and quality control is difficult. Oneevidence of this is the fluctuation in the mandrel pull test used totest the ability of wire coating to withstand stress cycles withoutexhibiting cracks or defects in the wire coating. Coatings formed usingthe conventional monoethanolamine approach to preparing the equilibratedpolyamide exhibits wide fluctuations in their ability to pass such testsand in many instances the wire is unacceptable.

Therefore, what is needed in the art is a method by which the polyamideresin may be equilibrated without the introduction or resultingby-products which will affect the quality of the final coated wireproduct.

DISCLOSURE OF INVENTION

The present invention discloses a magnet wire enamel composition whichis the reaction product of a polyamide resin, a solvolytic agent in thepresence of an organic titanate admixed with a blocked polyisocyanateand a hydroxyl rich polyester.

Also disclosed is a magnet wire substrate which is electricallyinsulated with one or more layers of the resultant enamel composition ofthe present invention.

Also disclosed is a method for applying the present enamel compositionto magnet wire substrate.

Further disclosed is a method of manufacturing the wire enamelcomposition of the present invention.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the preferredembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

A solution of the polyamide resin is prepared in the conventional mannerby dissolving the resin in a suitable organic solvent or mixture oforganic solvents. These solvents are generally conventional hydroxylcontaining aromatic hydrocarbons and some typical ones are cresylic acidand phenol. The preferred solvent is a mixture of cresylic acid andphenol, typically in concentrations of about 25 percent by weightcresylic acid to about 75 percent by weight phenol. It is preferred thatfor an enamel which is to be stored or is desired to have a stable shelflife, that the solvent contain greater than 60 percent by weight ofphenol.

The polyamide resin may be any polyamide resin useful as magnet wirecoatings. These are usually conventional nylon resins such as nylon 6;nylon 6,6; nylon 11; nylon 12 and aliphatic aromatic nylons such asTrogamide®. The preferred nylon is nylon 6,6 because of its high meltpoint. These nylons are commercially available and will have molecularweights (weight averaged) ranging above 25,000 and usually between25,000 to 40,000 and preferably between 25,000 to 35,000. Thedissolution of the resin in a solvent is performed using conventionaltechniques and equipment. Although almost any concentration of resin inthe solution may be possible to practice this invention, it is mostpractical to formulate the solution having a resin solids concentrationfrom about 10 percent to about 30 percent by weight with about 8 percentto about 15 percent by weight being preferred.

Once the polyamide resin has been dissolved, the solvolytic agents maybe introduced. These agents will be components particularly organicacids having at least one active hydrogen which may react with thepolyamide resin during the equilibration. The preferred agents are thealiphatic or aromatic alcohols such as ethylene glycol, however, othermaterials which may be used are adipic acid, hexamethylene diamine,water or the active hydrogen terminated polyamides and otherpolyalcohols. The solvolytic agent is generally added in amounts rangingfrom about 0.05 percent to about 15 percent by weight of the polyamideresin. It should be noted that the more solvolytic agent added relativeto the polyamide resin, the lower the molecular weight of theequilibrated resin will be and therefore, the lower the viscosity.Therefore, if one wanted to make a very low viscosity wire coating of apolyamide resin, one would add increasingly greater amounts of thesolvolytic agent. However, the preferred range is between about 0.05percent to about 5 percent by weight of the solvolytic agent to thepolyamide resin.

To this solution is then added an organic titanate catalyst such as achelate, alkyl or aromatic titanate. These titanates may be any of theorganic titanates, such as those sold by DuPont Corporation under thetrademark Tyzor® titanates or others. These titanates can be tetra-alkyltitanates having the formula

    Ti(OR).sub.4

where R is an alkyl group typically containing at least three carbonatoms. Examples of titanates such as these are tetra-isopropyl titanate,tetra-N-butyl titanate or tetrakis (2-ethyl hexyl) titanate. Otheruseful titanates can also be aromatic titanates having the formula

Ti(OR)₄

where R is an aromatic group and including polymeric titanates(typically dimers and trimers). Titanates of this variety are cresylic(or phenolic) titanates or their derivatives such as tetra-alkyl (e.g.ethyl, propyl etc.) phenolic, xylenolic, halogenated phenolic, etc. Thedimers and trimers will typically be linear (but may be branched) andchain extended through the --TiO-- group e.g. ##STR4##

However the preferred materials are the titanate chelates such as acetylacetonate titanate, ethyl acetonate titanate, etc. These materials willhave a structure ##STR5## in which X represents a functional groupcontaining oxygen or nitrogen and Y represents a two or three carbonchain, and R is an alkyl group typically containing at least threecarbon atoms. The most preferred titanate is the acetylacetonatetitanates having the structure ##STR6## In addition to these individualtitanates, mixtures of these titanates may also be used.

The titanate catalyst is added to the solution in concentrations ofabout 0.5 percent to about 5 percent by weight based on the weight ofthe resin present in the solution. The preferred concentration is about1 percent to about 2 percent by weight of the polymer resin. All of thereagents should be mixed to a homogenous solution using conventionalmixing or stirring apparatus. The solution is then heated to above itsreaction temperature which is typically above 100° C. and more typicallyin excess of 140° C. with the preferred range being about 140° C. toabout 160° C. The lower limit will be determined by the speed at whichthe reaction takes place (e.g. the lower the temperature the slower thereaction) while the upper temperature limit will be a function of thethermal stability of the reactants.

Although not wishing to be bound by any particular theory, it isbelieved that the hydrolysis takes place as a trans amide esterficationwhich takes place as shown below in reaction 4. ##STR7## wherein x+y=10to 200 and ##STR8## The reaction effectively cleaves the largermolecules of the resin in half thereby reducing the molecular weight inhalf as well. This results in a resin with significantly lowerviscosities at the same solids content. It has been discovered that theuse of the titanates are a critical feature of the equilibration processusing these solvolytic agents. It appears that the titanates act ascatalysts to advance the reaction at a speed which makes the reactionpractical in a production environment. It has been noted that thereaction remains at a virtual standstill irregardless of the temperatureuntil the introduction of the titanates. Once the titanate is added, thereaction advances at a reasonable pace. It has also been noted that themore titanate which is added to the reactants, the faster the reactionadvances.

In determining how far to allow the equilibration process to advancesamples are taken from the reaction vessel periodically. These samplesare then diluted to the predetermined final solids content desired usinga solvent mixture of phenol and cresylic acid (other compatible mixturesmay be used if desired), and viscosity measurements are made on thesamples. When the reaction material reaches the predetermined viscosityat a given temperature, the reaction is cooled to below 212° F. (100°C.). The solids content and viscosity of the desired final product will,of course, be a function of the enameling requirements of a given userand must be determined for each application.

To the resulting equilibrated polyamide resin solution is added ablocked polyisocyanate and a hydroxyl rich polyester. The blockedpolyisocyanate used may be any of the conventional blocked isocyanatesuseful in the production of polyurethane in the reaction with a hydroxylrich polyester. Although the isocyanate may be aromatic or aliphatic,the preferred material must be multi-functional. The most preferredmaterial is manufactured by Mobay Chemical Company and is called MobayDesmodur AP Stabil and is a blocked trifunctional isocyanate. Theresulting blocked isocyanate will unblock at temperatures compatiblewith the curing process of the enamel yet above the applicationtemperature of the enamel to the wire. This allows the enamel to bestable during preparation and application to the wire but react with thehydroxyl polyester when the coated wire is passed through the curingoven. Typically the curing oven will be about 750° F. or higher whilethe application process is carried out at temperatures below about 150°F. (65.5° C.). Therefore, these blocked isoyanates should reactaccordingly between these temperatures.

Typically, the polyisocyanates will be present in amounts from about 20to about 90 parts by weight per 100 parts by weight of the polyamideresin with the preferred amount being about 20 to about 50 parts byweight. More or less than these amounts can be added however, should toomuch polyisocyanate be added, insufficient cure of the enamel will takeplace and if too little isocyanate is added, the cured enamel will betoo brittle.

The hydroxyl rich polyester useful in practicing this invention arethose polyesters which are hydroxyl terminated, nonlinear polyestershaving molecular weights in the range of about 2,500 to 10,000 andpreferably between about 2,500 to about 5,000 (based on polystyrene asrun on gel permeation chromatography). The weight ratio between theblocked isocyanate and the hydroxyl rich polyester should be such thatthe ratio of isocyanate (NCO) to the hydroxyl groups of the polyester(OH) should be about 0.7 to 2.0.

These polyesters are typically present in the ratio described inrelationship to the polyisocyanate. Typically this will result in apolyester component being present at about 5 parts to about 50 parts byweight of the polyamide with about 15-40 parts by weight beingpreferred.

Once the materials have been added, and the solution prepared, it ismixed or stirred in a conventional manner until homogenous. Theresulting enamel may be adjusted for solids content and viscosity asdesired using compatible solvents to suit the particular enameling andcuring requirements of the wire enameling process to be used.

Thereafter these enamels may be applied using conventional wire enamelapplication to apply a thin film of enamel to the substrate andthereafter curing the enamel by exposure to heat.

EXAMPLE

An enamel of the present invention was prepared as follows:

To 143.3 pounds cresylic acid was added 48.4 pounds of reprocessed nylon66 fiber having an average molecular weight of 35,000 (weight averaged),0.1 pounds of tetra acetylacetonate titanate and 2.4 pounds of ethyleneglycol. This solution was agitated and purged with nitrogen gas at tenstandard cubic feet per minute (SCFM) for fifteen minutes and the flowrate was then reduced to 1 SCFM. The contents were then heated to atemperature of about 302° F.-311° F. (150° C.-155° C.). Agitation andnitrogen purge continued during the entire equilibration process. Afterthe solution had been at temperature for about 5 hours, samples wereremoved every two hours thereafter, thinned to a predetermined solidslevel, and tested for viscosity until such time as the viscosity reacheda previously determined set point of 160 centipoise at 86° F. (30° C.).Additionally, the solids contents of the solution was determined duringevery other sample. After the control point had been met, the heat wasturned off, the solution cooled to about 80° F. (26.6° C.) at which time113.5 pounds of cresylic acid and 112.4 pounds of xylene were added tothe solution and the temperture was again increased to about 212° F.(100° C.). To this solution was added 26.2 pounds of AP Stabil and 13.6pounds of hydroxyl terminated polyester, and while agitation wascontinued, the solution was mixed for 2 hours. After the material hadbeen at temperature for the 2 hours, the enamel was cooled to about 104°F.-122° F. (40° C.-50° C.), then filtered through a 5 micron filter. Theenamel had a final solids content of 16.1 percent, and a viscosity of160 centipoise at 86° F. (30° C.).

The enamel was then applied in a conventional manner by drawing the wirein contact with the enamel and forming a thin, smooth enamel coatingonto the copper wire (28 American Wire Gauge) which previously hadenamel basecoat applied to it at a thickness of 1.4 mils. This coatedwire was then passed through an oven thereby curing the enamel. Thetopcoat enamel was applied to a thickness of 0.5 mils. Additionally, anylon topcoat, which had been equilibrated using the traditionalmonoethanolamine and acetic anhydride process, was applied to similarwire having the same enamel basecoat. Each of the wires were tested forstress using the mandrel pull test and the results of the test are asindicated below:

The test comprises bending or wrapping the wire about a mandrel which isfive times the diameter of the bare wire, pulling the mandrel along thelength of the wire with a smooth rapid motion, and then inspecting theinsulation for cracking. The Table below shows the number of pulls whichthe individual wire withstood until the enamel cracked sufficiently tocause it to fail.

                  TABLE                                                           ______________________________________                                        Traditional nylon                                                                               1    10         7   4                                       Present Process nylon                                                                          12    15        13  15                                       ______________________________________                                    

As may be seen from the data, the equilibrated nylon topcoat developedusing the present process has been consistently high and acceptablewhile the prior art nylon coating is unpredictable and unacceptable foracceptability of a wire is 10.

The enameling composition offers a number of advantages to the prior artpolyamide systems. First the enamel will not contain any residual,reversible reaction products for the equilibration of the polyamideresin. Therefore, the resin will cure properly without interference fromthe ethanolamine and more uniform coating batches will be possible.

Secondly, the equilibration of the polyamide resin prior to reactionwith the polyester and the isocyanate, the resulting enamels will becapable of making higher solids contents enamels with lower workingvicosities. This will allow more enamel to be applied per pass usingconventional application techniques and will require less solvent,thereby reducing the cost.

Thirdly, as demonstrated in the test data, the urethane modifiedpolyamide coatings of the present invention have improved stresscapabilities over the modified polyurethane nylon enamels which were notsubjected to the present equilibration process.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritand scope of this novel concept as defined by the following claims.

I claim:
 1. A magnet wire having at least one layer of electricallyinsulating enamel comprising the reaction product of an equilibratedpolyamide resin, a blocked polyisocyanate and a hydroxylated polyesterwherein the equilibrated polyamide resin is the reaction product of asolvolytic agent having at least one active hydrogen and a polyamideresin in the presence organic titanate catalyst.
 2. A method of makingan electrically insulated magnet wire comprising drawing a magnet wiresubstrate through an enamel applicator, thereby applying a thin film ofmagnet wire enamel to said substrate in which the enamel compositioncomprises the reaction product of a polyamide resin, an organic titanatecatalyst and a solvolytic agent having at least one active hydrogenadmixed with a blocked polyisocyanate and a hydroxylated polyester,passing the coated magnet wire through a curing mechanism to cure theenamel.